The present invention relates to a power-driven screwdriver comprising:
a housing on which a depth stop can be fastened; PA1 a tool drive shaft, displaceable relative to the depth stop in the direction of its rotation axis, on which a tool receptacle is held; PA1 a drive shaft; PA1 a drive gear, received in rotatable and axially displaceable fashion on the drive shaft, which is motor-driven; PA1 an intermediate ring that is mounted rotatably on the drive shaft and has a first side facing toward the drive gear, and a second side; PA1 a cam ring that is mounted rotatably on the tool drive shaft and has a first side facing toward the intermediate ring, and a second side; PA1 first cam elements on the drive gear that coact with associated second cam elements on the first side of the intermediate ring in order to form a first cam clutch; PA1 third cam elements on the second side of the intermediate ring that coact with associated cam elements on the cam ring in order to form a second cam clutch; PA1 first catch elements on the intermediate ring that coact with associated second catch elements on the cam ring and form, together with the third and fourth cam elements, an entrainment clutch; and PA1 a first spring element for noiseless disconnection upon release of the release clutch. PA1 a throwout ring, which is mounted on the tool drive shaft against a resistance, is provided; PA1 the first spring element is arranged between the cam ring and the throwout ring in order to preload the cam ring in the direction toward the drive gear; PA1 a second spring element, supported on the housing side, is provided on the side of the drive gear facing away from the first cam elements; PA1 first claw elements, which coact with second claw elements on the throwout ring in order to form a disconnect clutch, are provided on the second side of the cam ring.
A screwdriver with depth stop of this kind is known from U.S. Pat. No. 4,655,103, the disclosure of which is fully incorporated by reference.
With the known screwdriver, a screw can be driven into a surface to a driving depth preset with a depth stop. When the depth stop encounters the surface, this initiates shutoff of a clutch with which a largely noiseless shutoff is accomplished. A motor-driven drive gear, which together with an idler gear arranged in axially movable fashion forms a first cam clutch, is provided for this purpose. The idler gear coacts, on its other side facing the tool carrier, with a further clutch element, the oblique cam surfaces of the associated elements forming a second cam clutch. In addition, catch elements in the form of straight, axially parallel flanks, by way of which, when a certain baseline torque occurs, the clutch element joined to the tool receptacle is entrained by the intermediate clutch element, are provided on the idler gear and on the second clutch element. In addition, a compression spring by way of which the idler gear is preloaded in the direction toward the tool receptacle is arranged between the drive gear and the idler gear.
In operation, first the depth stop is set to the desired driving depth and then the tool carrier with its tool is placed onto, for example, a screw that is to be driven in, and pressed down. As a result, all three clutch elements come into engagement with one another, so that initially the torque is transferred from the drive gear to the tool carrier as the screwdriving operation begins. As a result of the torque occurring during the screwdriving operation, the idler gear and the clutch element joined to the tool receptacle are pushed slightly apart until the straight catch flanks come into engagement with one another and positive entrainment is guaranteed. When the depth stop encounters the surface, the tool carrier with the tool receptacle and the screw move even further until the latter is completely driven in. The cam elements of the first cam clutch then slide apart, assisted by the compression spring, until it releases. Because the torque has now decreased to zero, the idler gear is pressed by the compression spring against the second clutch element that is joined to the tool receptacle, so that the drive gear can continue to rotate freely without touching the idler gear. A "noiseless" shutoff is thus accomplished.
Also known are various screwdrivers which provide for a disconnection of the drive train by means of a shutoff system as soon as a preset torque is reached (cf., for example, EP 0 239 670 B1).
It is also known, in the case of a screw machine tool, to implement selectably a switchover capability between a shutoff by way of a depth stop and a shutoff by way of a preset torque (EP 0 401 548 B1). If this screwdriver is used for shutoff with a depth stop, the construction and operation then correspond in principle to the aforementioned U.S. Pat. No. 4,655,103.
By way of a spring-loaded coupling ring that is activated via pins upon removal of the depth stop and that covers the intermediate ring and the second clutch element facing the tool drive shaft, the intermediate clutch element and the second clutch element are pressed toward one another, when the depth stop has been removed, under the action of the spring, and are positively joined to one another via a spline set, so that these two elements act like a single clutch element even when acted upon by torque. What therefore results in this position, in combination with the oblique cam surfaces of the drive gear, is a cam clutch that releases at a torque that is preset by way of a corresponding adjustment mechanism.
Even after release, however, this clutch continues to run and "chatter."